The present invention relates to a spectrometer for examining the optical properties of a substance.
If a substance is irradiated with light, it will absorb the optical energy. This energy will be converted into atomic vibrations, e.g., heat by a non-radiation transition, except that consumed either for another light emission (photoluminescence) or for photo-chemical reactions. The photo-acoustic spectrometry is defined to measure the thus generated calorie or the accompanying strain as a function of excitation optical energy. For this photo-acoustic spectrometry, there is known a device as shown in FIG. 7. This method is disclosed in the reference of the Bul. Electrotech Lab. Vol. 47 No. 2(1983). A sample 5 is placed in a sealed container 3 having an optical irradiation window 1 and an acoustic detector (or a highly sensitive microphone) 2 and is irradiated from the outside with interrupted monochromatic light 6. If the sample 5 absorbs the light and heats up, the temperature of gases surrounding the sample 5 will also rise. As a result, the gas layer is expanded to act as a piston thereby to generate pressure waves 7 in the sealed container 3. If, moreover, the irradiation light is cyclically interrupted, the pressure waves change into sound waves. When the wavelength of the irradiation light is changed and the amount of heat resulting from absorption of light for each wavelength is measured by the microphone system, the absorption spectrum of the sample 5 is obtained. This is known as the gas microphone method for measurement of the photo-acoustic spectrum. There is also known a piezoelectric element method by which strain waves 8 induced in accordance with a local heat generation are directly detected by the use of a piezoelectric element 9 or the like, as shown in FIG. 8.
The gas microphone method described above according to the prior art is accompanied by the following problems: The sealed container for placing the sample has to be filled up with some gas for propagating the deformation of the sample as the pressure waves to the microphone so that the sample being measured has its surface contaminated with the molecules of the gas, whereby the measurements cannot be accomplished with a clean sample surface or in a vacuum; and a substance having a small optical absorption constant is difficult to measure because of limited sensitivity since the pressure waves caused by the deformations of the sample are detected by the microphone.
In order to improve the resolution in the depth direction, on the other hand, it is necessary to increase the interrupted (or modulated) frequency thereby to shorten the period of time for the heat propagation. In the gas microphone method, the signal is weakened with the frequency f of interruption in the form of f.sup.-1 or f.sup.-3/2 so that the interruption frequency f cannot be increased so much. In the piezoelectric element method, on the other hand, the light has to be interrupted (or modulated) with the resonance frequency of the element so as to improve the detection sensitivity so that the modulation frequency cannot be continuously changed to limit the depth resolution.